1. Field of the Invention
The present invention relates to a method of mounting a chip, and specifically to a method of mounting a chip to a substrate at a fluxless condition while applying an underfill agent between the chip and the substrate,
2. Discussion of the Related Art
A method of mounting a chip is well known, wherein bumps are formed on the chip by a solder and the like, the chip is approached to a substrate in a form of face down, and after the bumps are brought into contact with electrodes of the substrate, the bumps of the chip are heated and melted to be bonded to the electrodes of the substrate. An underfill agent, which is composed of a nonconductive adhesive, is injected into a gap between the chip and the substrate in order to increase the total bonding strength between the chip and the substrate. As this underfill agent, usually an adhesive comprising a thermosetting resin is used. The reliability of the electrical insulation between the bonded portions is increased by a condition where the underfill agent is filled around the bumps of the chip and the electrodes of the substrate bonded to each other.
In a conventional method of mounting a chip formed with solder bumps, firstly a flux is applied to the bump side or the substrate side, and after the chip is mounted on the substrate at a predetermined position, the solder bumps are melted by heating (in most cases, associated with pressing) and the bumps are preliminarily bonded to the electrodes of the substrate. After being cooled, the flux is removed by cleaning, and thereafter, an underfill agent is injected into a minute gap between the chip and the substrate from the side direction. After the underfill agent is injected, heating is performed again, the underfill agent is re-flown and thereafter cured.
In such a conventional mounting method, however, when the solder bumps are heat-melted in the preliminary bonding process of the chip and the substrate, there is a fear that the bonded portions are secondarily oxidized by the surrounding atmosphere. If such a secondary oxidation occurs, it may decrease the reliability of the electrical bonding between the bumps and the electrodes of the substrate. In order to prevent the secondary oxidation, there is a method for purging the atmosphere around the bumps using nitrogen gas and the like when the bumps are bonded to the electrodes. However, such purging by nitrogen gas and the like causes a chip mounting apparatus to become large-sized, the mounting apparatus and the mounting process to become complicated, and the cost thereof to increase, and because it is necessary to take a time for the purging by nitrogen gas and the like, a high-speed mounting may be obstructed.
Further, in the above-described conventional mounting method, because it is necessary to apply a flux and remove the flux by cleaning after the bonding for melt-bonding of the solder bumps, there is a problem that the number of processes until completion of main bonding is fairly great. Moreover, the perfect cleaning of the flux is difficult, and therefore, the residual components may reduce the reliability of the bonding. In order to achieve a fluxless bonding, proposed is a method for using an alcoholic organic material having a melting point higher than that of a solder instead of a conventional flux, and removing it by vaporizing without cleaning it after the preliminary bonding (JP-A-8-293665). In this method, however, basically only the cleaning process of the flux becomes unnecessary, and the method is poor in effect for reducing the number of the processes in the bonding. In particular, the processes for injecting an underfill agent after the preliminary bonding, for re-flowing the underfill agent and for curing the underfill agent still remain as they are. Further, in this method, as the bump pitch has been in a fine pitch condition, it has become difficult to inject the underfill agent.
With respect to such a problem, although there is no description as to fluxless condition, a method, wherein an adhesive resin (an underfill agent) is spread and filled between a substrate and a chip in which specially shaped bumps each having a small-diameter tip are formed and the bumps are bonded to the electrodes of the substrate by press-breaking the tips of the bumps of the chip, is disclosed in JP-A 11-26506.
However, the method disclosed in JP-A 11-26506 basically does not aim a method for bonding bumps to electrodes of a substrate by melting the bumps by heating. For example, if this method is applied to mounting of a chip accompanying with melting the bumps by heating, in a case where a conventional resin is used as the underfill agent, when a low-viscosity resin is used and the underfill agent is to be spread by pressing between a chip and a substrate, the underfill agent flows out, the underfill agent cannot be held so that the bumps and the electrodes press-contacted to each other are covered with the agent, and voids may be involved. On the other hand, when a high-viscosity resin is used, the bumps melted by heating cannot be thoroughly wet-spread. Therefore, the underfill agent may be nipped between the bumps and the electrodes, a compound between metals required for bonding may not be formed between the bumps and the electrodes, a resistance of the bonded portion may increase, or a sufficient bonding strength may not be obtained, and therefore, the reliability of the bonding decreases.
Further, because the underfill agent heated together with the bumps starts to be cured in a short period of time, the bumps do not thoroughly wet-spread by being suppressed in flowability by the resin starting to be cured, an enough bonding state also cannot be obtained. Further, in a conventional resin prepared as the underfill agent, if the resin viscosity is tried to be decreased as lower as possible such that the molten bumps can thoroughly wet-spread, the curing time becomes long, and it becomes difficult to shorten the tact time of the production process. Furthermore, because the conventional resin requires a long time from the start of curing to a curing degree enough for holding the bonding state of bumps and electrodes, the chip must be continued to be pressed until the curing progresses sufficiently, and it is also a great obstruction factor for shortening the tact time of the production process.